Scanning probe microscopy is born with the invention of the scanning tunneling and the atomic force microscope. In brief, it aims at forming images of sample surfaces using a physical probe. Scanning probe microscopy techniques rely on scanning such a probe, e.g. a sharp tip, just above or in contact with a sample surface whilst monitoring interaction between the probe and the surface. An image of the sample surface can thereby be obtained. Typically, a raster scan of the sample is carried out and the probe-surface interaction is recorded as a function of position. Data are thus typically obtained as a two-dimensional grid of data points. The resolution achieved varies with the actual technique used: atomic resolution can be achieved in some cases. Typically, either piezoelectric actuators or electrostatic actuation are used to execute precise motions of the probe. Two main types of SPM are the scanning tunneling microscopy (STM) and the atomic force microscopy (AFM).
In AFM techniques, forces between the tip and the surface are monitored; these are notably the short range Pauli repulsive force (in contact-mode) and the longer range attractive force (in non-contact mode, e.g., the van der Waals forces). Using AFM techniques, imaging of the surface topology is usually carried out in one of three modes: contact mode, non-contact mode and intermittent contact or tapping mode. In the contact mode, the probe is moved over the surface with constant contact thus monitoring the surface by changing the height set-point. In the non-contact (or “noncontact”) mode, a stiff cantilever oscillates with a small amplitude of typically less than 10 nm above the surface. Influences of the surface lead to changes in frequency and amplitude of the cantilever. These changes can be detected and used as the feedback signals. In the tapping mode, the cantilever is oscillated with a larger amplitude. Therefore also short range forces are detectable without sticking of the cantilever to the surface. Still, the short range forces can be measured also in non-contact AFM, even using small amplitudes, e.g., using the so-called ‘qPlus sensor’, a tuning fork or a length extension resonator.
The above techniques are translated into topography by means of a sensor. A common type of sensor is a bulk-component-based free-space laser beam deflection setup with a four quadrant photo diode acting as the deflection sensor. Other known principles include piezoelectric, piezoresistive and thermal height sensing deflection sensors. In both STM and AFM, the position of the tip with respect to the surface must be accurately controlled (e.g., to within about 0.1 Å) by moving either the sample or the tip. The tip is usually very sharp; ideally terminated by a single atom or molecule at its closest point to the surface.
Kelvin probe force microscopy (or KPFM, also known as “surface potential microscopy”) and the closely related “electrostatic force microscopy” (EFM) are noncontact variants of AFM. KPFM measures work function differences or the surface potential (for non-metals) between a conducting sample and a vibrating tip, at atomic scales. Note that the concept of work function breaks down on the atomic scale; the measured values correspond to the contact potential difference (“local contact potential difference” or LCPD). The LCPD is the figure to be determined by KPFM. It has been shown that the LCPD varies on the atomic scale. The potential offset between the probe tip and the surface is measured using the cantilever as a reference electrode that forms a capacitor with the surface over which it is scanned, to obtain a map. Note that it does not have to be at constant separation (and usually is not). The LCPD corresponds to the voltage applied to the sample (with respect to the tip) that yields the minimal frequency shift Δf(V). Two methods are known to find this minimum (i.e., the LCPD). The first method (voltage spectroscopy) consist of slowly sweeping the voltage, i.e., measuring the Δf(V) relation and determining its minimum (voltage spectroscopy). The second method requires modulating the voltage (applying and oscillating an AC component plus a DC component) to find the DC voltage that yields the minimum frequency shift. The second method often employs lock in technique; often the second Eigen-resonance of the cantilever is used as frequency for the AC component, thus taking advantage of the cantilever's quality factor.
The electrostatic force microscopy (or EFM) directly measures the force produced by the electric field of the surface on a charged tip. In EFM, the frequency shift or amplitude change of the cantilever oscillation is monitored to detect the electric field. Still, EFM and KPFM are often regarded as a same general noncontact variant of AFM. Both EFM and KPFM require the use of conductive cantilevers, typically metal-coated silicon or silicon nitride.
KPFM and EFM become increasingly important for the characterization of molecular and atomic scale electronic functional structures as they provide a measure of the electrostatic field and/or Work functions. Therefore, such techniques are sensitive to charges and charge distributions. Charge resolution of about 0.1 electron charges combined with lateral resolution on the atomic scale was recently demonstrated in [3]. In particular, high resolution KPFM is foreseen to be of great importance when studying and developing future single electron logic devices or novel materials exploiting charge transfer, e.g. materials for OLEDs and materials for organic solar cells. However, to obtain high resolution maps as presented in [3], measurement times are on the order of a day for one KPFM map, corresponding to ˜5000 measurement points (or pixels).
The two different techniques commonly used to measure KPFM maps are compared in [2]. Essentially, the first technique (voltage spectroscopy) provides high resolution but is slow, while the second technique (using lock-in technique) is fast but lacks resolution.